1. Field of The Invention
This invention relates to capacitors which exhibit good capacitor characteristics such as frequency characteristics and dielectric strength characteristics and more particularly, to small-sized, large capacitance, solid electrolytic capacitors of the type which comprises at least one conductive polymer layer doped with a mixed dopant composed of a polyvalent anion and a monovalent anion. The invention also relates to a method for making such capacitors as mentioned above.
2. Description of The Prior Art
The digitalization of electric and electronic appliances which has been recently in progress requires capacitors of the type which is small in size and great in capacitance with a low impedance in a high frequency range.
Conventional capacitors which have been in use in a high frequency range include, for example, plastic capacitors, mica capacitors and layer-built ceramic capacitors. However, these capacitors are so large in size that it is difficult to realize a great capacitance.
Capacitors of a large capacitance type include, for example, electrolytic capacitors such as aluminum dry electrolytic capacitors and aluminum or tantalum solid electrolytic capacitors. These capacitors have a very thin oxide film serving as a dielectric and can thus realize a large capacitance. However, such a very thin oxide film is very liable to suffer damages. In order to repair the damages, it is necessary to form an electrolyte layer on the oxide film. The electrolyte layer serves also as a true cathode.
For instance, with aluminum dry capacitors, a liquid electrolyte is used such that anodic and cathodic aluminum foils which have been, respectively, etched on the surfaces thereof are convolutely wound through a separator, in which the liquid electrolyte is impregnated. The liquid electrolyte exhibits ionic conductivity and is so great in specific resistance that a great loss is incurred, thereby presenting the problem that the resultant capacitor is great in loss and very poor in frequency characteristics of impedance and temperature characteristics. In addition, the use of the liquid electrolyte inevitably involves the leakage and evaporation of the electrolyte, coupled with another problem that the capacitor undesirably decreases in capacitance and increase in loss as time passes.
Tantalum solid electrolytic capacitors make use of a manganese oxide electrolyte and can overcome the problems on the variations of temperature characteristics, capacitance and loss in relation to time. However, the manganese oxide electrolyte is relatively high in specific resistance. This leads to the frequency characteristics of impedance which are poorer than those of layer-built ceramic capacitors or film capacitors.
Additionally, the formation of the manganese oxide electrolyte layer essentially requires repetition of several to several tens of cycles of the step of immersion in a manganese nitrate solution and subsequent thermal decomposition at a temperature of about 300.degree. C., thus being complicated in the formation step.
In recent years, another type of solid electrolyte capacitor have been proposed wherein after formation of a conductive layer such as of a metal, a conductive metal oxide or a conductive polymer such as polypyrrole on a dielectric film, another conductive polymer such as polypyrrole is further formed via the first-mentioned conductive layer by electrolytic polymerization (Japanese Laid-open patent Application Nos. 63-158829, 63-173313 and 1-253226).
Further, large-capacitance film capacitors have also been proposed wherein the dielectric thin film made of an electrodeposited polyimide is formed on an etched aluminum foil on which conductive polymer films are successively formed by chemical polymerization and electrolytic polymerization, respectively, thereby forming an electrode (The technical Report at the 58th Meeting of the Electrochemical Society, pp. 251 to 252 (1991)).
However, where an electrolytically polymerized polymeric product is formed through the conductive, thermally decomposed metal oxide such as manganese dioxide, the dielectric film is very liable to be damaged during the course of the thermal decomposition. In order to obtain a capacitor having a high dielectric strength, it becomes necessary to repair the dielectric layer prior to the electrolytic polymerization, thus presenting the problem that the procedure becomes more complicated.
As having set out hereinabove, with the tantalum solid electrolytic capacitors, the electrolyte composed of manganese oxide is formed by repeating the thermal decomposition cycle. To repair the film damaged during the repetition of the thermal decomposition cycle requires the formation of a dielectric film after every cycle of the thermal decomposition. This also involves the problem of complicating the procedure.
Moreover, where a conductive polymer layer is formed by chemical polymerization, it has been found difficult to form the conductive polymer layer at a high packing rate to the depth of irregularities of an etched aluminum foil or to the depth of fine pores of a tantalum sintered body. Additionally, the use of an electrolytically polymerized product having high conductivity may lead to the lowering of the dielectric strength of the resulting capacitor.